The present invention relates generally to radiotelephone systems. More particularly, the present invention relates to methods and apparatus for establishing direct communication among physically close mobile stations in a radiotelephone system using Opportunity Driven Multiple Access protocol.
A variety of radiotelephone systems have been developed for providing communication service between mobile stations and base stations. Radiotelephone systems generally include cellular telephone systems, personal communication systems (PCS), trunked radio systems and other similar radio systems. In such a system, one or more base stations provide two-way radio communication service to mobile stations operated by subscribers and located in the geographic area served by the base station. As a mobile station is powered up or newly moves into a base station's coverage area, the mobile station registers with the base station, including identifying itself and its capabilities. As a mobile station moves among geographic areas, communication between the mobile station and a base station is handed off to other base stations. Such systems typically also include a mobile switching center to control handoff operations and traffic routing in the system.
Radiotelephone systems permit radio communication of voice and data between mobile stations and between mobile stations and the public switched telephone network. However, mobile-to-mobile communications have heretofore been completed through a base station. To initiate a call to another mobile station in the system, a calling subscriber at an originating mobile station initiates a radio link with its serving base station, which is generally the geographically closest base station. The originating mobile station sends a call request to the base station. The call request includes identification information such as a mobile identification number for the called mobile station, or terminating mobile station.
The base station then locates the terminating mobile station in the system. The base station may consult with the mobile switching center to learn with which base station the terminating mobile station last registered. If the terminating mobile station is registered with the base station itself, the base station sends a page to the terminating mobile station. The page is a radio communication with predetermined timing and content inviting the terminating mobile station to respond to begin two-way communication. The terminating mobile station then responds and the base station completes the call between the calling subscriber at the originating mobile station and the called subscriber at the terminating mobile station. Typically, either the calling subscriber or the called subscriber or both are billed for the time during which the call is underway.
In current systems, this process is followed even if the two mobile stations are in close physical proximity, close enough for reliable radio contact. The two-way communication between the two mobile stations requires two two-way radio links. A first link is maintained between the originating mobile station and the base station. A second link is maintained between the base station and the terminating mobile station.
In cases where mobile stations are physically close together, this is wasteful of system resources. If direct mobile to mobile communication was possible, the two base station radio links would be freed up for use by other mobiles, improving utilization of system resources. Moreover, the mobiles could operate at reduced transmit power, reducing radio frequency noise in the system for other base stations and other mobiles. Still further, since system resource usage is reduced by removing the base station from the communication link, alternative pricing options could be offered for subscribers.
Accordingly, there is a need for improved method and apparatus for mobile to mobile communication in a radiotelephone system.